Abstract

The liquid dominated Ulubelu geothermal field is
situated in Lampung, southern Sumatra, Indonesia.
The field lies in steep terrain between 300 and 1600 m
above sea level (a.s.l.) but mostly from 700 to 800 m.
The host rocks are mainly Quaternary andesitic
pyroclastics and lavas. The field is at the southern end
of the Great Sumatra Fault. The basement comprises
pre-Tertiary volcanic rocks. Samples from 3 wells,
namely UBL-01, UBL-02 and UBL-03, were studied
and geophysical data, made available by Pertamina
including gravity and resistivity results, were used.
Hydrothermal minerals display a distinct zonation of
both clay and non-clay minerals. They show that the
rock-altering fluid was produced from a liquid
dominated reservoir. There are four hydrothermal
mineral zones, namely: smectite, mixed-layer clays,
chlorite and chlorite-epidote. The smectite zone is
present at shallow levels (from near surface to 650 m
a.s.l.). The mixed-layer clay zone occurs at 500 to 650
m (a.s.l.). The chlorite zone occurs at 100 to 500 m
(a.s.l.). The chlorite-epidote zone occurs deeper than
100 m (a.s.l.).
The measured formation temperatures are generally
lower than those indicated by the mineral
geothermometers (chlorite, epidote, wairakite and
others) by about 20o to 60oC and the fluid inclusion
geothermometers by 10o to 100oC. Cooling may have
occurred although the measured well temperatures
may be too low, as wells were not thermally stable
when the measurements were made.
The permeability of the reservoir is indicated by the
presence of albite and adularia. These minerals mostly
indicate higher permeable zone and coincide with the